The present invention relates to a composite material made by coating or impregnating a heat-resistant fabric base with tetrafluoroethylene resin and baking the resin, and to a process for manufacturing the same.
As is well known, tetrafluoroethylene resin (hereinafter referred to as "PTFE") has excellent chemical resistance, heat resistance, electrical insulation, self-lubrication and non-adhesiveness, and is finding wide applications in various industrial fields. But, because of these properties, it is difficult to process.
PTFE starts to melt at 327.degree. C., but does not fluidize even above the melting point. So, unlike ordinary thermoplastic resins, it cannot be molded by screw extrusion, injection molding or rolling molding.
One conventional method for forming a baked layer of PTFE on a heat-resistant fabric was to coat or impregnate the fabric with PTFE in the form of powder, aqueous dispersion or paste by applying or immersion, and bake the PTFE. In this method, it has so far been customary to coat a heat-resistant fabric with an aqueous dispersion of PTFE to a thickness of 20 microns or less by applying or immersion, dry at about 90.degree. C. for about 5 minutes, and bake it at 370.degree.-400.degree. C. for 10 to 20 minutes. If it is desired to form the layer to a larger thickness, the abovesaid steps are repeated until a predetermined thickness is reached.
However, if PTFE is baked for a long time, the fabric substrate is also subjected to a high temperature so that its strength is decreased. Glass fiber, for example has a heat resistance of about 600.degree. C. or more. But, by heating at 300.degree. C., 350.degree. C. and 400.degree. C., its strength is diminished to about 72%, 57% and 42% of the original strength, respectively. For all-aromatic polyamides, by heating at 300.degree. C., 350.degree. C. and 400.degree. C., its strength is diminished to about 48%, 30% and 22%, respectively.
Thus, the conventional method for making a composite material having a fabric base with a baked layer of PTFE has a shortcoming that baking impairs the strength of the heat-resistant fabric substrate. With the conventional method, a composite material having a sufficient strength cannot be obtained.